1. Field of the Invention
This invention relates to an observation optical system, and in particular, to an observation optical system in which an ordinary forward observation is made and at the same time, a backward observation can also be made.
2. Description of Related Art
As conventional observation optical systems in which the forward observation and the backward observation can be made at the same time, optical systems set forth, for example, in Japanese Patent Kokai Nos. 2002-33943 and 2000-322564 are available.
The optical system set forth in Kokai No. 2002-33943 is related to an omnidirectional visual sensor. This optical system includes a rotating mirror and a wide-angle lens. The rotating mirror directs a convex surface toward the image pickup side and has a notch at its center. The wide-angle lens is provided in the notch of the rotating mirror. It is possible to image a front visual field through the wide-angle lens and an ambient visual field including a rear visual field through the rotating mirror by using a common image pickup means.
The optical system set forth in Kokai No. 2000-322564, like the optical system of Kokai No. 2002-33943, is related to an omnidirectional visual sensor. This optical system includes a wide-angle lens and a curved-surface-shaped reflecting mirror. The wide-angle lens is provided to image visual information in horizontal and upward directions. The curved-surface-shaped reflecting mirror is placed at the lower portion (on the image pickup side) of the wide-angle lens and is used to image visual information in horizontal and downward directions. The visual information obtained through the wide-angle lens and the curved-surface-shaped reflecting mirror is imaged using the common image pickup means.